Long time constant eddy current compensation

Abstract

Magnetic coils (12) create a generally uniform magnetic field through an examination region (10). A radio frequency transmitter applied RF pulses to an RF antenna (26). A gradient field controller (30) generates gradient excitation current pulse profiles which pass through an eddy current calibration circuit (50) to gradient coils (38). A radio frequency receiver (92) receives magnetic resonance signals from the examination region, which signals are Fourier transformed (94) into an image representation (96). The eddy current calibration circuit (50) modifies the current pulse profiles in order to compensate for eddy currents, i.e. to remove the effect of eddy currents such that the resultant gradient magnetic field has a selected profile. An automatic calibration circuit (60) adjusts the decay constant or center frequency of a band pass filter (56) and the gain of an amplifier (58). A sine wave generator (62) applies a sine wave voltage to the gradient coils as a search coil monitors (64) the resultant magnetic field and a current probe (70) monitors the current flowing through the gradient coil. The magnetic field and current are phase sensitively detected (66, 72) to determine the relative amplitude and phase of each. The relative amplitude and phase are compared as the frequency of the sine wave signal is swept. The time constant or center frequency of the band pass circuit is set to the frequency (78) at which the field phase lags the current phase by a maximum and the gain is set in accordance with a fractional fall-off (f) in the relative amplitude.

Having thus described the preferred embodiments, the invention in which exclusive right and proprietary interest is claimed is as follows: 
     
       1. A method of calibrating an eddy current correction circuit for a gradient coil, which circuit has at least a time constant or center frequency adjustment and a gain adjustment, the method comprising: applying an oscillating electrical signal to the gradient coil;   monitoring at least amplitude and phase of a resultant magnetic field generated by the gradient coil;   sweeping the applied oscillating signal through a range of frequencies;   determining a frequency about which at least one of the amplitude and phase of the monitored field changes relative to the applied signal;   setting a center frequency of the correction circuit in accordance with the determined frequency.   
     
     
       2. The method as set forth in claim 1 further including: determining a fractional fall-off of the relative amplitude of the applied signal and monitored field adjacent the determined frequency; and,   adjusting the correction circuit gain in accordance with the determined fall-off.   
     
     
       3. The method as set forth in claim 1 further including the step of quadrature phase detecting the monitored magnetic field to determine the amplitude and phase thereof. 
     
     
       4. The method as set forth in claim 3 wherein the determining step includes: determining a lag between the applied signal and magnetic field phases; and,   monitoring the phase lag as a function of frequency to determine the frequency at which the phase lag is a maximum.   
     
     
       5. The method as set forth in claim 4 further including: determining a ratio of the amplitude of an applied signal and magnetic fields; and,   determining a fractional fall-off in the amplitude ratio in a frequency range across the determined frequency; and,   setting the correction circuit gain in accordance with the predetermined fractional fall-off.   
     
     
       6. The method as set forth in claim 5 further including monitoring current flow through the gradient coil and determining the applied signal amplitude and phase therefrom. 
     
     
       7. The method as set forth in claim 1 wherein the determining step includes determining a plurality of frequencies at which the amplitude and phase of the monitored field changes and further including storing each of the frequencies and at least the corresponding determined phases and amplitudes of the monitored changes relative to the applied signal phases and amplitudes. 
     
     
       8. The method as set forth in claim 7 further including: preforming a least square analysis of the stored phases and amplitudes; and,   adjusting a corresponding plurality of correction circuit centre frequencies and gains in accordance therewith.   
     
     
       9. A method of correcting for eddy currents in an apparatus for applying gradient magnetic field pulses across an examination region, which apparatus includes a gradient field control means for generating current pulses wit a selected profile, an eddy current correction circuit including a plurality of parallel connected correction paths, each path including a band pass filter having an adjustable time constant and an amplifier means having an adjustable gain, the gradient current correction circuit being operatively connected between the gradient field control means and at least one coil for causing magnetic field gradients in the examination region, the correction circuit altering each current pulse profile passing from the gradient field control means to the coil to cause the resultant magnetic field gradient to conform more closely to the unaltered current profile, the method comprising: applying an oscillating signal of an adjustable frequency through the correction circuit to the coil to create a corresponding oscillating magnetic field in the examination region;   determining changes in the relative amplitude and phase of the applied oscillating signal and the resultant magnetic field as the frequency of the oscillating signal is swept over a preselected frequency range;   adjusting the time constant of at least one of the band pass filters in accordance with the monitored relative phase change; and,   adjusting the gain of at least one of the amplifier means in accordance with the relative amplitude change.   
     
     
       10. In an apparatus for applying gradient magnetic field pulses across an examination region, which apparatus includes a gradient field control means for generating current pulses with a selected profile, an eddy current correction circuit including a plurality of parallel connected correction paths, each path including a band pass filter having an adjustable center frequency and an amplifier means having an adjustable gain, the gradient current correction circuit being operatively connected between the gradient field means and at least one coil for causing magnetic field gradients in the examination region, the improvement comprising: an oscillator for applying an oscillating signal to the gradient coil;   a search coil which is selectively disposed in the examination region to monitor the resultant magnetic field;   a phase sensitive detector operatively connected with the search coil for producing first signals indicative of the amplitude and phase of the monitored magnetic field relative to the applied oscillating signal;   a maximum phase lag frequency determining means operatively connected to the phase sensitive detector for determining at least one frequency at which the phase of the magnetic field lags the coil current by a local maximum; and,   a time constant adjusting means for adjusting the center frequency of one of the band pass filters in accordance with each local maximum phase lag frequency.   
     
     
       11. The apparatus as set forth in claim 10 further including: a gain determining means operatively connected to the comparing means for determining a fractional fall-off in amplitude of the magnetic field relative to the applied signal over frequencies adjacent at least one local maximum phase lag frequency; and,   a gain adjusting means for adjusting the gain of one of the amplifier means in accordance with each fractional fall-off.   
     
     
       12. The apparatus as set forth in claim 10 further including: a current probe for monitoring current flow through the gradient coil;   a second phase sensitive detector operatively connected with the current probe for producing second signals indicative of the amplitude and phase of the current flowing through the gradient coil relative to the applied oscillating signal;   a comparing means for comparing the first and second amplitude and phase signals from the first and second phase sensitive detectors, the comparing means being operatively connected with the maximum phase lag frequency determining means.   
     
     
       13. The apparatus as set forth in claim 12 wherein the comparing means compares the monitored current phase and the monitored magnetic field phase and wherein the maximum phase lag frequency determining means plots the phase lag vs. a log of the frequency and determines the frequency at which the phase lag is a maximum. 
     
     
       14. The apparatus as set forth in claim 12 further including: a means for determining a relative amplitude shift between the monitored current and magnetic field adjacent the determined phase lag frequency, the relative amplitude change determining means being operatively connected with the comparing means; and,   a gain adjustment means for adjusting the amplifier means gain in accordance with the determined amplitude change.   
     
     
       15. The apparatus as set forth in claim 14 wherein the comparing means divides the first amplitude by the second amplitude and wherein the amplitude change determining means plots the amplitude ratio vs. a log of the frequency. 
     
     
       16. The apparatus as set forth in claim 10 further including: a main magnetic field means for generating a generally uniform magnetic field through the examination region;   a radio frequency transmitter for applying radio frequency pulses to the examination region;   a sequence control means for causing the radio frequency transmitter and the gradient pulse control means to apply a sequence of radio frequency and magnetic field gradient pulses to the examination region to induce phase and frequency encoded magnetic resonance;   a receiving means for receiving the phase and frequency encoded magnetic resonance signals from the image region; and,   a Fourier transform means for Fourier transforming the received magnetic resonance signals into an image representation.